In life, the creature probably resembled a chimpanzee more than anything else. It moved through a lakeside landscape of grasslands and forest searching for food, accompanied by small bands of its fellows, most likely, and keeping a sharp eye out for pythons, crocodiles and saber-toothed cats. This animal probably shared the forest with apes and monkeys and, like them, spent some time up in the trees. It may have walked upright, which apes rarely do for very long at a stretch. But at a casual glance, it would have seemed to our eyes like just another chimp.

In death, however, this creature has just sent shock waves through the world of science. After eight grueling years of hunting in the hot, wind-scoured desert of central Africa, an international team of researchers has uncovered one of the most sensational fossil finds in living memory: the well-preserved skull of a chimp-size animal, probably a male, that doesn't fit any known species. According to paleontologist Michel Brunet of the University of Poitiers in France, whose team reported the find in Nature last week, there is no way it could have been an ape of any kind. It was almost certainly a hominid  a member of a subdivision of the primate family whose only living representative is modern man. And it has left scientists gasping with astonishment for several reasons.

To start with, it is nearly 7 million years old  a million years more ancient than the previous record holder. Indeed, this new species is as much older than the famous Lucy as Lucy is older than we are. It almost certainly dates from very near that crucial moment in prehistory when hominids began to tread an evolutionary path that diverged from that of chimps, our closest living relatives. Even more surprising, this ancient hominid was not discovered anywhere near the Great Rift Valley of East Africa, where all the record setters of the past three decades have been found. Instead, it turned up in the sub-Saharan Sahel region of Chad, more than 1,500 miles to the west, forcing a rethinking of the conventional wisdom about where humans arose.

Beyond that, the animal's habitat may cast further doubt on the already beleaguered notion that our ancestors first emerged on a treeless savanna. It now looks as though this pivotal event happened in a setting that was at least partly wooded. Most remarkable of all, though, is the skull itself. The creature, known formally as Sahelanthropus tchadensis (roughly translated "Sahel hominid from Chad") and informally as Toumai ("hope of life," in the local Goran language), has a mix of apelike and hominid features. And to some paleontologists, the hominid features, especially the face, are a lot more modern-looking than anyone would have expected at so early an evolutionary stage. "A hominid of this age," writes Bernard Wood of George Washington University, in a commentary accompanying the Nature articles, "...should certainly not have the face of a hominid less than one-third of its geological age."

Paleontologists are scrambling to digest the implications of this remarkable find. It may simply be that the discovery fills in the evolutionary sequence  the so-called family tree  that leads to modern humans. But some argue that the new fossil might represent something far more revolutionary. It could entirely demolish the idea of a tree, replacing it with something more akin to a thick, bristly bush. Many scientists now believe that the emergence of humans may not have been the neat succession of increasingly modern-looking ancestors suggested by conventional hominid family trees but rather an evolutionary brawl, with multiple species fighting for survival  and the survival of their gene pool  at just about every point in prehistory. No matter what the answer, says Daniel Lieberman, a biological anthropologist from Harvard, "this is one of the most important fossil discoveries in the past 100 years."

Given the harsh conditions that exist today at the excavation site, a place called Toros-Menalla, it's a wonder that the fossil was found at all. This brutally hot, dune-spattered landscape lies in the middle of the Djurab Desert, about four days' drive north of N'Djamena, the capital of Chad. Rusting pieces of battle tanks are scattered all about, leftovers from 30 years of civil unrest. Windstorms blow up frequently, sandblasting everything in sight  including the scientists, who have to seek refuge inside their tents for days at a time. "They really are the most unimaginable field conditions," says Harvard paleontologist David Pilbeam, who has worked with Brunet.

Dozens of spectacular fossils of human ancestors had previously been discovered in the Great Rift Valley, part of which slices from the Red Sea through Ethiopia, Kenya, Uganda and Tanzania, but Brunet had reasons to suspect that Chad could also be fruitful. For one thing, the area around Lake Chad had already yielded a rich trove of primitive vertebrate fossils. For another, he knew that the Rift Valley's paleontological track record did not actually prove that humanity had arisen only there. And there was a nice practical advantage as well. As a Frenchman, Brunet was especially well equipped by language and history to work in a former French imperial outpost like Chad.